/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        ListNode dummyNode = new ListNode(-1);
        ListNode head = dummyNode;

        while(l1 != null && l2 != null) {
            if (l1.val <= l2.val) {
                head.next = l1;
                l1 = l1.next;
            } else {
                head.next = l2;
                l2 = l2.next;
            }
            head = head.next;
        }

        if (l1.next != null) {
            head.next = l1.next;
        }

        if (l2.next != null) {
            head.next = l2.next;
        }

        return dummyNode.next;
    }

    public ListNode mergeTwoLists2(ListNode l1, ListNode l2) {
        ListNode dummyNode = new ListNode(-1);
        dummyNode.next = l1;

        ListNode head = dummyNode;

        while(l2 != null) {
            if (l1 == null) {
                head.next = l2;
                break;
            }

            if (l1.val <= l2.val) {
                l1 = l1.next;
            } else {
                ListNode tmp = head.next;
                head.next = l2;
                l2 = l2.next;
                head.next.next = tmp;
            }
            head = head.next;
        }

        return dummyNode.next;
    }
}